The assignee of the present invention manufactures and deploys communications satellites. A large number of such satellites have been employed in varying orbits for a variety of missions, including point to point communications of voice and/or data. A system of such satellites providing communications service to mobile users on a substantially worldwide basis is termed a Global Mobile Satellite System (GMSS). Examples of deployed or proposed GMSS's include Globalstar, ICO, Inmarsat, Iridium, Orbcomm, and Teledesic.
GMSS's have been proposed and deployed for use in both low earth orbit (LEO) and geostationary orbit (GSO). Satellites in GSO operate at an orbital altitude approximately forty-five times higher than a typical LEO altitude; as a result, for a given desired data rate, a communications link between a GSO satellite and a terrestrial user requires higher power and/or a larger antenna than a communications link between a spacecraft in LEO and a terrestrial user. A GSO satellite operates over the earth's equator, and can provide only limited coverage to regions at latitudes substantially higher than 70 degrees. However, a GSO satellite is substantially stationary relative to any point on the earth's surface, and may provide communications coverage to an area representing as much as 40% of the earth's surface, whereas each single spacecraft in LEO is able to service only a much smaller geographic footprint, and has a substantial motion with respect to any point on the ground.
Taking into account a perceived market need for telephone and low data rate service for mobile users in locations underserved by terrestrial cellular telephone services, presently operating LEO GMSS's such as Iridium and Globalstar, and GSO GMSS's such as Inmarsat, were deployed to provide such services. Adequate voice service, and low data rate data services, typically less than 5 Mbps, are available to subscribers over these services.
A typical GMSS 100 is illustrated in simplified form in FIG. 1. The system includes a network of satellites 101(1)-101(n). Each satellite 101(i) is communicatively coupled, either directly or by way of one or more crosslinks 113, to at least one terrestrial gateway 102 and a plurality of user terminals 106. The user terminals 106 comprise satellite terminals that may be handheld mobile telephones or car phones, or terminals installed on mobile and/or remote platforms such as ships, aircraft, buoys, and the like. The satellites making up the GMSS 100 may be in LEO or GSO, as an example, but may also operate in medium earth orbit (MEO), highly inclined elliptical orbits (HIEO), or some combination of all or some of these orbits or other earth orbits so as to provide substantially global coverage.
A terrestrial gateway 102 communicates with a satellite 101 over a feeder link 103; a user terminal 106 and a satellite 101 communicate over a user link 107. Traffic generated by a subscriber 106 may be routed from satellite 101(1) in view of subscriber 106 to a satellite 101(n) in view of gateway 102(m) by way of one or more crosslinks 113. Communications traffic initially received by multiple satellites 101 may be delivered via one or more crosslinks 113 to satellite 101(n), which may downlink the traffic to gateway 102(m) over a feeder link 103. Outbound traffic to a user 106 as well as command and control communications can follow the opposite path: gateway 102(m) may uplink the traffic to satellite 101(n) over feeder link 103, and satellite 101(n) may forward the traffic to the user 106 and one or more other satellites 101(i) via one or more crosslinks 113.
A GMSS 100 provides low data rate services, as well as voice services, for users over user link 107. User link 107 conventionally operates at a relatively low frequency, for example in the L-band. Feeder link 103, on the other hand, is conventionally operated at a relatively high frequency, for example in the Ka-Band.
Corresponding to the above-described system architecture, and referring now to FIGS. 2a and 2b, each satellite 101 includes an earth pointing main mission antenna (MMA) 201, operable (at L-band, for example) to complete user link 107; a feeder link antenna 202 operable (at Ka-band, for example) to complete feeder link 103; and a crosslink antenna 203 operable (also at Ka-band, for example) to complete cross link 113. MMA 201 may be communicatively coupled at a low data rate (e.g., <5 Mbps) to one or more user terminals 106. Received communications traffic is processed onboard by digital communications processor 205 and routed according to the intended destination of the traffic.